A great deal of mechanical energy is stored in the deformed limbs of a drawn crossbow. That energy is applied to the crossbow by the mechanical work done by the archer when the crossbow is drawn. Some crossbows include a stirrup at the front end that is arranged to be placed on the ground and held down by the archer's foot while he or she pulls the bowstring. Other crossbows include one or more cranks, pulleys, levers, or other mechanical aids to draw the crossbow. In either case, once drawn, the bowstring is held in the drawn position by a hook, caliper, or other retainer until released by triggering the crossbow. The mechanical energy stored in the deformed limbs is converted (mostly) to kinetic energy of the bolt shot by the crossbow.
Once drawn, it is difficult to release the energy stored by the drawn crossbow without shooting the bolt. Releasing the drawn bowstring without a bolt in place is ill-advised; without the bolt to take up the pent-up energy of the drawn crossbow, that energy instead often results in damage to the crossbow or injury to the archer. Mechanical aides used for drawing the bowstring typically are not arranged to operate in reverse (i.e., to enable controlled “un-drawing” or “de-cocking” of the crossbow). If a safe area is available where the bolt can be fired, then the bolt can be shot by the crossbow into that area. However, that usually results in loss of the bolt due to the long range of the crossbow or damage to the bolt upon striking an obstruction (e.g., a tree or the ground).